1. Field of the Invention
The invention relates to improved bleaching compositions and methods for removing hydrophobic stains from fabrics.
2. The Prior Art
Peroxygen bleaches are well known for their commercial usefulness in facilitating stain and/or soil removal from fabrics. Hydrogen peroxide is the most common peroxygen bleach. Although very effective on a variety of stains, hydrogen peroxide requires relatively high activation temperatures and long wash times, e.g. greater than 60.degree. C. for more than 30 minutes. A continuing trend toward lower wash temperatures has presented a need for peroxygen bleaches with efficacy at temperatures of 40.degree. C. and less.
One class of peroxygen bleaches that are particularly effective are organic peracids chemically depicted as RCO.sub.3 H. The structure of R greatly affects reactivity, solubility and surface activity of a given peracid. Hence, the bleaching efficacy of peracids on stained laundry articles varies greatly depending, through R, on the peracid's relative hydrophobicity or hydrophilicity. For instance, alkyl peracids with chain length greater than about 7 carbon atoms are effective on hydrophobic as well as hydrophilic stains. On the other hand, alkyl peracids with shorter chain length are only effective on hydrophilic stains. Aromatic peracids such as perbenzoic acid are intermediate, i.e. they bleach hydrophobic stains but to a lesser extent than the alkyl peracids. As a result of their potent reactivity, it is difficult to stabilize many peracids so as to directly formulate them with a detergent powder or even as a separate bleach additive product. However, the peracid bleach benefit can be delivered by incorporating into the cleaning powder a two component bleach system, which upon dissolution in the wash liquor react to generate the aforementioned peracid. These systems consist of a source of hydrogen peroxide, such as sodium perborate, and a peracid bleach precursor or activator. Common precursors are found in the class defined by substituted and unsubstituted carboxylic acid esters having a water soluble leaving group.
U.S. Pat. No. 2,955,905 (Davies et al.) is one of the earlier patents in the field revealing this technology. Davies et al. discloses several classes of esters including the commercially available benzoyl ester of sodium phenol sulfonate. Therein, it is suggested that the proportion of ester to per-salt may range in the ratio of 1/4 to 2 molecules ester per 1 atom of available oxygen and having present an alkaline material to given an initial pH of between 9 and 11 in the aqueous bleaching solution.
Another early patent of interest is GB No. 864,798 (Hampson et al.) which under the same pH and persalt to reactive ester molar proportions improved upon Davies et al. by recognizing enhanced storage stability with use of acylated phenol esters such as p-acetoxybenzene sulfonate.
U.S. Pat. No. 4,412,934 (Chung et al.) urges the ratio of peroxide source to precursors be at least 1.5 and preferably greater than about 3, to realize maximum conversion of precursor into the reactive peracid. Therein is taught that hydrogen peroxide to precursor ratios of 1 or less results in a lowering of bleaching performance. Below a molar ratio of 1.5, there was found to be a competing chemical reaction diminishing the amount of percarboxylic acid in favor of diacyl peroxides said to perform quite poorly. A preferred pH range was also found to lie between 9 and 10.
The concept that excess hydrogen peroxide over precursor in molar amounts greater than 1.5:1 must be present has become an established principle found in a wave of subsequent patents. These patents include U.S. Pat. No. 4,536,314 (Hardy et al.) and EP No. 0 163 331 (Burns et al.).
U.S. Pat. No. 4,671,891 (Hartman) instructs on compositions that can bleach a wide variety of different types of stains. To obtain removal of both tea and tomato stains, it was found necessary to utilize a halogenated peroxybenzoic acid and a carbonyl carbon atom containing activator which together form diacyl peroxides. The molar ratio of peroxycarboxylic acid to bleach activator covers a range from about 10 to 0.05. These compositions were also said to be highly pH dependent broadly ranging from 6 to 12 but optimally between 8.0 and about 10.
With the exception of the Hartman patent, most of the known art focusing on precursor and sodium perborate achieves bleaching of only certain types of stains. Most often, the foregoing systems are able to cope with hydrophilic stains, such as tea, but are quite poor at eliminating hydrophobic stains such as generated from tomato sauce. The approach in U.S. Pat. No. 4,671,891 reports a more broad based stain removal but accomplishes this at high cost since it involves use of expensive peroxy carboxylic acids in addition to expensive activators.
Consequently, it is an object of the present invention to provide a bleaching composition that is effective at removing a wide range of stains including those of the hydrophobic and hydrophilic variety.
Another object of the present invention is to accomplish removal of a wide range of stains with as simple and economical a system as possible.
These and further objects of the invention are more fully illustrated by reference to the detailed discussion and examples that follow.